This invention relates to communication systems and in particular to a communication system which is capable of transmitting binary data in a reliable and efficient way.
As is well known, based on prior communication techniques, there is a great deal of art related to the transmission of binary data. Essentially, the prior art was concerned with the effective bandwidth required to transmit binary data with minimum error. There have been various schemes which serve to conserve bandwidth as related to the transmission of binary data over conventional communications links. In any event, in order to transmit binary data, it is known that the transmission bandwidth in such systems has to be as wide as the effective bandwidth of the data that is being transmitted. Thus, the prior art employed conventional types of transmission procedures including amplitude modulation, frequency modulation and several other techniques.
Also according to the prior art, single sideband suppressed carrier modulation was employed to transmit the binary data in an effort to reduce the effective bandwidth. Utilizing such single sideband transmission techniques as those techniques employing band pass filtering and suppressed carrier modulation, one had to restore the carrier in exact phase and frequency. These systems required that there be minimal phase shift in the band pass to avoid data distortion. The prior art suffered from many disadvantages in the use of single sideband transmission systems and such systems are not widely employed for the efficient transmission of such data except over "High Frequency" or "Short Wave" circuits.
In regard to transmission systems, a very well known technique is employed for rating transmission efficiency. The rating scheme was devised by Nyquist and is known as the Nyquist 1, 2 Criterion. The efficiency is expressed in bits per cycle of bandwidth. Some typical figures utilizing this rating are as follows. For a base band NRZ (non-return to zero) system, there is a Nyquist Criterion of 2. When one employs a modulated carrier such as an amplitude modulated carrier, this system has a Nyquist Criterion of 1. In a single sideband carrier system where one employs only one sideband, the Nyquist Criterion is 2.
There is a signal encoding technique which is referred to as MFM or Miller Encoding. This encoding technique is widely used in computer disc recording to narrow the bandwidth required by the head and media. The system uses a series of rules to determine when a voltage change is to be made. For example, during a 1, the voltage will change at the center of the bit. During a 0, the voltage will change at the end of the bit. When a 1 follows a 0, the change is delayed until the center of the 1. The overall format employed in the MFM or Miller Encoding System employs bit widths of 1, 1.5 and 2.0. Ignoring the spectral content of the square corners in the frequency analysis, the time periods involved are 2, 3 and 4 times the clock period. In an ideal situation, only those frequencies resulting from those periods need to be used since the original wave form can be restored from them. This is a encoding technique which results in a considerable bandwidth savings and as indicated, is widely employed. Thus, in an MFM single sideband system where one employs one sideband, the Nyquist Criterion can be made to equal 4 with suitable filtering. In a quaternary single sideband system, which is a four phase system, the Nyquist Criterion is also equal to four.
As one can see from the above, the MFM single sideband system or the quaternary SSB system affords a transmission improvement of 2 times over base band NRZ or the single sideband carrier system, while it gives an improvement of 4 times over the typical AM modulated carrier system. As will be described, the present invention enables one to achieve a Nyquist Criterion of 6 utilizing and employing the techniques to be described and relating to the present invention.
It should be obvious from the above description that single sideband transmission combined with improved encoding, for example as the MFM system and filtering, greatly improves the efficiency of a communication system. Since a narrower bandwidth is also employed, it will also improve the signal to noise ratio. As will be explained, the system described herein, which employs a unique encoding technique, utilizes periods of 2.0, 2.5 and 3.0 times the clock period so that a band pass filter can be narrowed more than that utilized in an MFM system. Thus, an efficiency which is 50% greater is achieved since the period difference is 2-3 instead of 2-4.
As will be explained, the system to be described herein utilizes three unique features which are not known to be incorporated in any other communication systems and which enables the system to operate more efficiently and more effectively than other prior art systems. The system to be described has particular relevance to telemetric systems including FM/SSB subcarriers and also has applications in sending digital data on FM broadcast station subcarriers, through narrow band communication receivers, local area networks and data over voice telephone exchanges. Thus, the system to be described employs a newly conceived technique that enables data to be sent at efficiencies of 6 bits per cycle of bandwidth (Nyquist efficiency). The system is usable in all radio and cable systems, but has a primary use for FM/SCA systems where up to four high speed data channels can be transmitted within a single FM station subcarrier space. These channels can be 38.4, 19.2, 19.2 and 19.2 KB channels offering several times the utilization capability of the SCA spectrum when compared to a 38.4 KB FM/FM system. Alternatively, four audio and one high speed data channel can occupy the same space. As will be further explained, it is also usable in communication radio systems where 38.4 KB data can be passed through a standard 455 KHz IF system bandwidth.
Essentially, a main advantage of the system to be described is the complete elimination of the carrier. The carrier is completely eliminated at the transmitting end and does not have to be recovered at the receiving end. This technique alone results in a great savings of bandwidth and also reduces receiver complexity. Unlike conventional single sideband suppressed carrier systems, carrier reinsertion is not required and all the disadvantages of such systems are circumvented. As is well known in single sideband binary data transmission systems, there are many disadvantages associated with the same. The first disadvantage is that there is low frequency distortion based on the isolation of one of the sidebands and there is a great deal of distortion which is provided by carrier reinsertion for coherent detection at the receiver. The concept of transmitting a signal without a carrier has been investigated in the prior art. See for example, U.S. Pat. No. 3,835,386 issued on Sept. 10, 1974 entitled Binary Data Communication Apparatus and issued to Frederick C. Court. This patent describes a system for single sideband transmission of binary data. The apparatus comprises a coder which converts a binary data sequence to a related code sequence. There are gate circuit means with a carrier signal generator and a band pass filter. The related code sequence and carrier signal are applied to the gate circuit so that the related code sequence gates the carrier signal through to the output. The output of the gate circuit comprises bursts of carrier signals. The gate circuit is further arranged so that alternate bursts of carrier signals are in anti-phase relationship with each other. These carrier signal bursts are applied to a band pass filter. The response of the filter is such that when a half bit duration burst is applied to it, the output is a burst of oscillations of twice the bit duration rising from 0 amplitude to maximum during the first bit period and falling again to 0 in the second bit period. The filter's output is an amplitude dipolar angle modulated signal which only occupies a single sideband and purportedly can be detected at the receiver by a simple envelope detector. There are a great many difficulties encountered when one tries to employ this technique.
The difficulty with this system is that the filter means as described offers a uniform phase shift with frequency. Thus, the patent indicates a phase shift occurring with time which is absolutely contrary to the fact that such filters do not exist. In order for the system as described in that patent to function, one must have 180.degree. phase shift differences at all frequencies involved. This is also not obtainable. The modulation means creates signals varying in frequency according to the relationship F.sub.c +F.sub.m, where F.sub.m is equal to 1/T, where T is the time between pulses. Hence, to pass the lowest and highest frequency required, F.sub.c must be at one edge of the filter and F.sub.c +F.sub.m (the highest value of F.sub.m) at the other edge. It is possible utilizing only one value of F.sub.m to obtain a 180.degree. phase shift, but not all values.
Depending upon the number of zeros transmitted, F.sub.m will vary from 0 HZ upwards. Therefore, the 0 signal crossings predicted in the patent do not occur. Furthermore, while a filter at the transmitter might give 180.degree. phase shift at one frequency, the transmission path requires a plurality of filters. These filters will destroy all phase relationships as created at the transmitter. Thus, any total phase difference of 30.degree. or more from the ideal will completely destroy the signal as far as 0 envelope crossings are concerned rendering the system inoperative or extremely difficult to implement. Hence, as indicated, there is a substantial problem involved with such systems and therefore such systems have not been commercially exploited or employed.
As will be described, in the present invention, data is pre-encoded in a narrow band width format as in a MFM or other format. The data as applied to the modulator is NRZ of bit widths equal to or greater than one bit width. The system employs a standard balanced modulator which is commonly used and of the type which suppresses the carrier as will be explained. The system requires a special filter having 0.degree. phase shift across its passband, which is unique, and enables one to remove all signs of the carrier in conjunction with the modulator. The pre-encoded and the modulator filter removes all lower base band signals thus requiring a bandwidth 1/4 or less than that required by a double sideband communication system. Because perfect zero crossovers and perfect 180.degree. phase changes are not achievable and therefore envelope detection cannot be used, phase detection means is employed by the system resulting in a reduced bandwidth by a significant factor, which results in more efficient use of the radio spectrum. Because of the narrow bandwidth, the signal to noise ratio is greatly improved resulting in an increased range and improved noise immunity.
The system employs further techniques to achieve further noise reduction. The detector means is not synchronous or any way dependent upon the carrier frequency. Envelope zero crossings are not essential at all. Phase differences other than 180.degree. are completely detectable and in practice, differences of plus or minus 45.degree. from the ideal do not result in any significant errors. The system is centered on detectable phase changes and has no need to set a particular clipping level as is taught in the above-noted reference. In fact, another difficulty regarding the system as described above is that it will not function if a plus or minus 45.degree. variation in phase from the ideal were received.
It also should be apparent as one can ascertain from the prior art, that the majority of systems which have been described and discussed, require a synchronous carrier detector means in order to regenerate the carrier at the receiving site, which this system does not.